1. Field of the Invention
The present invention relates to a head for a thermally assisted magnetic recording that records data by emitting near-field (NF) light on a magnetic recording medium and by decreasing an anisotropic magnetic field of the magnetic recording medium and to a head gimbal assembly and a magnetic recording device that uses such head.
2. Description of the Related Art
In the field of magnetic recording using a head and a medium, further improvements have been demanded in performance of thin film magnetic heads and magnetic recording media in view of an increase in recording density of magnetic disk devices. For the thin film magnetic heads, composite type thin film magnetic heads configured from lamination of a reading magnetoresistive (MR) element and a writing electromagnetic conversion element are being widely used.
The magnetic recording medium is a non-continuous medium, in which magnetic microparticles are aggregated. Each magnetic microparticle has a single magnetic domain. In this magnetic recording medium, a single recording bit is configured by a plurality of magnetic microparticles. Therefore, to increase magnetic density, the size of the magnetic microparticles must be reduced, and asperity at a border of adjacent recording bits needs to be minimized. However, if the size of the magnetic microparticles is reduced, there is a problem that thermal stability for magnetization of the magnetic microparticles is lowered as the volume of the magnetic microparticles is reduced.
To address this problem, increasing magnetic anisotropic energy Ku of magnetic microparticles may be considered. However, this increase in Ku causes an increase in anisotropic magnetic field (coercive force) of the magnetic recording medium. On the other hand, the upper limit of the writing magnetic field intensity for the thin film magnetic head is determined substantially by saturation magnetic flux density of a soft magnetic material forming a magnetic core in the head. As a result, when the anisotropic magnetic field of the magnetic recording medium exceeds an acceptable value determined from the upper value of the writing magnetic field intensity, writing becomes impossible. Currently, as a method to solve such a problem of thermal stability, a so-called thermally assisted magnetic recording method has been proposed, which, using a magnetic recording medium formed by a magnetic material with large Ku, performs the writing by heating the magnetic recording medium immediately before applying the writing magnetic field to reduce the anisotropic magnetic field.
For this thermally assisted magnetic recording method, a method that uses a near-field light probe, a so-called plasmon generator, which is a piece of metal that generates near-field light from plasmon excited by emission of laser light, is known.
A magnetic recording head provided with a conventional plasmon generator has a configuration in which a main pole is provided on a trailing side of a near-field light generating portion of the plasmon generator and in which a waveguide that propagates light is provided so as to oppose the plasmon generator. This plasmon generator excites surface plasmon by coupling with the light that propagates through the waveguide in surface plasmon mode and generates near-field light at the near-field light generating portion as a result of propagation by this surface plasmon propagating through the plasmon generator. Furthermore, a magnetic recording medium is heated by the near-field light generated at the near-field light generating portion of the plasmon generator, an isotropic magnetic field of the magnetic recording medium is reduced, and thereby information is written. However, with a magnetic recording head having this configuration, after the temperature rises due to heating, the magnetic field is also applied to the magnetic recording medium during the cooling process. Accordingly, after the application of the magnetic field for recording is completed, the magnetic field is further applied even onto the magnetic microparticles where the magnetization has not yet stabilized. Therefore, there is a problem that sufficient signal-to-noise ratio (S/N ratio) cannot be obtained at high recording density.
Therefore, in order to achieve high recording density and obtain a sufficient S/N ratio, a configuration, in which a magnetic field is applied prior to heating the magnetic recording medium, is conceivable. Or in other words, that is a configuration where the plasmon generator of a conventional magnetic recording head is provided on the trailing side of the main pole. A magnetic recording head as described in Japanese Patent Publication No. 2010-244670 has been proposed as a magnetic recording head with this configuration. With this magnetic recording head, it is thought that almost no magnetic field is applied to the magnetic recording medium during the cooling process, therefore, rapid magnetization reversal is possible in the adjacent magnetic domains on the magnetic recording medium, and that the requirements for high recording density and sufficient S/N ratio can be satisfied. However, the light spot diameter of the near-field light irradiated on the magnetic recording medium by the near-field light generating portion is currently being required to be even smaller because of demand for even higher recording density in recent years.